----------------------------------------------------------- Minutes of the ABP-RLC team meeting of 24.06.2005 present: UD, AG, WH, EM, TP, FR, FZ excused: EB web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/ ------------------------------------------------------------ (1) Minutes of last meeting, pending actions, announcements ----------------------------------------------------------- The RLC team went through the list of actions at http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/Actions/actions.htm . The comparison of various implementations of the Burov-Lebedev formulae is completed, thanks to EM. The question of the slot impedance for TCDQ and TCDS seems also settled. We will not request any hardware modifications, since the possible improvement is at the most 15% of a contribution which amounts to about 1% of the total impedance. Numerical results by AG suggest that the slot impedance scales as -2.5th power of the beam-wall distance. FR suggested that the scaling could be derived from a two-step approach, where the first step consists in the excitation of the radiating slot and the second step describes the action back on the beam. AG remarked that the TDI is similar to the TCDQ/TCDS, and also close to the beam with slots on both sides. Some changes to the TDI design were suggested by him a while ago (changes to the tank, and connecting absorber blocks to beam screen). AG has asked for the modified drawings, but he has received no response yet. TP pointed out an error in the last week's minutes. The COMBI comparisons are made with simulations by Ji Qiang and not Shi. The minutes will be corrected. FR announced that the recommendations of the last LHC MAC have been published (They are attached to the RLC minutes web site). The conclusions on the electron cloud appear correct and they are consistent with our view. They rectify a possibly false impression left at an earlier LTC presentation. Other questions mentioned in the MAC report are the tolerances on beta beating in the collimation system, and the installation of a physics coordinator, who will, e.g., channel and limit the optics requests. FR reported that the MKI kicker impedance was measured for the first time by Fritz Caspers and Tom Kroyer. The impedance is of order 1-3 Ohm up to 500 MHz. A comparison should be made with and without coating of the ceramic chamber. Presently a discussion is ongoing as to whether the coating should be in the form of longitudinal stripes (preferred solution of FC) or in the form of a helix (proposed by BT group in order to reduce the risk of sparking). FR emphasized that establishing a complete LHC impedance model is of high priority, and that the team should be prepared to respond to Lyn Evans' questions within a day and not after 1 week. Another topic which may come up for discussion soon is the emittance budget and the detuning with amplitude at injection. (2) Follow-up of head-tail growth rates vs Q' (EM) -------------------------------------------------- EM presented single-bunch and coupled-bunch head-tail growth rates (and tune shifts) for a wider range of Q' values. Only a small additional tune spread would be required in order to avoid instability for positive Q' values. For large positive values, the beam is always stable, which may explain SPS observations, where a large positive chromaticity is used to combat electron cloud without leading to head-tail instabilities. The dependence of the coupled-bunch growth rate on the mode number is somewhat different from FZ's presented in the previous meeting. Possible reasons could be a different impedance model and a different counting of the multibunch modes. In EM's case, the most critical mode has the mode number M-[Qy+1], and it is dominated by the resistive wall low-frequency impedance. FR commented that additional impedances could change the picture at large Q' substantially (Q'=10 corresponds to a frequency shift of about 500 MHz) and that a complete impedance model would be needed. Beam-beam effects at collisions generate additional tune spread of order 0.001 (WH), which could stabilize the LHC beam. This could be taken into account in deciding the optimum injection scheme. WH and LV wrote a note on the stabilization by beam-beam about 2 years ago; see LHC Project Note 316. These estimates should be repeated for LHC injection conditions. FR and WH pointed out that, for LHC, injection oscillations and injection beta beating should be considered. TP and FZ reported an observation by Wolfram Fischer of beam-beam induced beam losses when injecting the last bunches into RHIC. (3) AOB ------- FR remarked that the AB department is in charge of machine protection only, and that this is the only task of the collimation system. Responsible for the background in the experiments is E. Tsesmelis. An ABP group meeting will be held next week Wednesday 29.06.2005 at 10:30. FZ gave a brief summary of the London "BLT" (ILC BDIR, ELAN, EUROTeV) joint workshop. Nick Walker and Jean-Pierre Delahaye became deputy regional ILC directors, who will assist the European director Brian Foster in the ILC design. G. Blair has expressed strong interest in sending UK graduate students to CERN for ~1 year periods, for work on LHC, LHC upgrade, CTF-3 or CLIC. FZ presented an ATF-2 commissioning strategy and EUROTeV electron-cloud plan & progress, He also drafted two work plans for ILC WP3 task forces 6 and 7 (electron-cloud and fast-ion effects, respectively.) Posted on the web: Slides by EM Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/